My invention relates to a frequency-shift-keyed receiver, and particularly to such a receiver for use in power line carrier systems and other noisy environments.
Frequency-shift-keyed signals are used extensively in various communication systems. One such use is in 60 Hertz power line systems where line conditions are indicated by frequency-shift-keyed signals. For example, a first power station may supply power over a transmission line to a second station. The second station monitors its loads and other outgoing lines, and as long as these loads and lines are in good condition, the second station sends a first frequency guard signal to the first station. As long as the first station receives this guard signal, it supplies power over the line to the second station. But if the second station detects an improper condition, such as a fault on one of its outgoing lines, the second station removes the guard signal and sends a second frequency trip signal to the first station. Upon loss of the guard signal and upon receipt of the trip signal, the first station trips (or opens) the transmission line to the second station, thereby protecting the equipment at the first station. Because of modern society's need for electrical power, the decision to maintain or open an electrical power line is very important.
Accordingly, a primary object of my invention is to provide a new and improved frequency-shift-keyed receiver for use in the control of electrical power lines.
Another object of my invention is to provide a new and improved frequency-shift-keyed receiver.
Usually, the guard and trip signals are sent and received by carrier frequencies over telephone lines or radio circuits. But whatever facility or circuit may be used, its connection or utilization at a 60 Hertz power station makes it inherently noisy from an electrical view-point. And when there is a 60 Hertz power or equipment malfunction, the communication facility or circuit may become even noisier, just at the time reliable guard and trip signals are most needed. While telephone lines or circuits are reliable, they may be subjected to alien tones by accident or by cross-talk. If such an alien tone were at or close to the trip frequency, a false trip might occur.
Accordingly, another object of my invention is to provide a new frequency-shift-keyed receiver that provides improved reliable operation even under very noisy electrical conditions.
Another object of my invention is to provide a new frequency-shift-keyed receiver that can detect the presence of two tones (guard and trip) at the same time, and prevent the trip tone from causing a trip function if the guard tone is present.
Frequency-shift-keyed receivers for power line trip and guard signals have been provided in the past. However, the ability of the prior art receivers has been limited because the desirable characteristics of the receiver made them relatively complicated and expensive.
Accordingly, another object of my invention is to provide a new and improved frequency-shift-keyed receiver that is relatively simple but that performs well and reliably.
Another object of my invention is to provide a new and improved frequency-shift-keyed receiver that can reliably indicate receipt of a first or guard frequency signal, a second or trip frequency signal, or both guard and trip frequency signals at the same time.
In one prior art frequency-shift-keyed receiver, crystal filters were used to detect receipt of the first frequency signal or the second frequency signal. Such crystal filters are relatively expensive, and if the guard and trip frequencies must be changed, then additional crystal filters are required for that receiver.
Accordingly, another object of my invention is to provide a new and improved frequency-shift-keyed receiver that does not require crystal filters, and that can have its frequency of operation changed relatively easily.